Add schedule interval as attribute of a program

Also, the part of the schedule event is deleted and the length of the event types
changes to the nearest suitable after changing the length of the interval.

This closes #1220

app/serializers/event_serializer.rb

@@ -26,6 +26,6 @@ class EventSerializer < ActiveModel::Serializer
   end
 
   def length
-    object.event_type.try(:length) || EventType::LENGTH_STEP
+    object.event_type.try(:length) || object.event_type.program.schedule_interval
   end
 end